1. Field of the Invention
The present invention relates to a zoom lens, which is suitable for an image pickup apparatus such as a digital still camera, a video camera, a TV camera, or a monitoring camera.
2. Description of the Related Art
In recent years, an image pickup apparatus (camera) such as a video camera or a digital still camera using a solid-state image pickup element has a smaller size and more sophisticated functions. Further, along with the smaller size and more sophisticated functions of the image pickup apparatus, an imaging optical system used for the image pickup apparatus is required to be a small zoom lens having a wide angle of field (photographing angle of field), a large aperture ratio, and high optical performance. As a zoom lens having a small entire system, a wide angle of field, and a large aperture ratio, there is known a negative lead type zoom lens in which a lens unit having a negative refractive power is positioned in front (disposed closest to an object side).
Japanese Patent Application Laid-Open No. 2001-141997 discloses a three-unit zoom lens including, in order from the object side to an image side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a positive refractive power, in which an interval between the lens units is changed for zooming. Japanese Patent Application Laid-Open No. 2001-141997 discloses a zoom lens having a relatively small size and a larger aperture ratio as an F-number of approximately 2.3 at a wide angle end compared with an F-number of approximately 2.8 that is common in conventional systems. In addition, Japanese Patent Application Laid-Open No. 2008-065051 discloses a two-unit zoom lens including, in order from the object side to the image side, a first lens unit having a negative refractive power, and a second lens unit having a positive refractive power, in which an interval between the lens units is changed for zooming. Japanese Patent Application Laid-Open No. 2008-065051 discloses a zoom lens that has achieved a large aperture ratio as an F-number of approximately 1.4 at the wide angle end.
In recent years, the zoom lens that is used for the image pickup apparatus is strongly required to have a large aperture ratio and high optical performance over the entire zoom range.
The negative lead type zoom lens described above can relatively easily achieve a wider angle of field and a smaller size of the entire system. However, because the entire lens system becomes asymmetric with respect to the aperture stop, variations of aberrations due to zooming become larger when the aperture ratio is increased, and hence it is difficult to obtain high optical performance over the entire zoom range. In order to achieve a smaller size of the entire system and a larger aperture ratio in the negative lead type two-unit zoom lens or three-unit zoom lens described above, it is important to appropriately set a lens structure of each lens unit constructing the zoom lens and a position of the aperture stop disposed in an optical path.
For instance, it is important to appropriately set the lens structure of the second lens unit and move the aperture stop for zooming. When these structures are inappropriate, it becomes very difficult to obtain high optical performance while achieving downsizing of the entire system and a larger aperture ratio.
The three-unit zoom lens disclosed in Japanese Patent Application Laid-Open No. 2001-141997 has an F-number of approximately 2.3 at the wide angle end, which is not sufficient as a larger aperture ratio. On the other hand, the two-unit zoom lens disclosed in Japanese Patent Application Laid-Open No. 2008-065051 has a large aperture ratio as an F-number of approximately 1.4 at the wide angle end, but downsizing thereof is not sufficient because the first lens unit includes four lenses.